1. Field of the Invention
The present invention relates to a lithium secondary battery using a transition metal chalcogenide or a lithium.cndot. transition metal chalcogenide as an active material for negative electrode.
2. Description of Related Art
With recent development of portable equipment such as personal computers, portable telephones, etc., demand for batteries as electric sources thereof much increases. Particularly, lithium batteries are intensively investigated in various fields as batteries capable of providing high energy density because lithium has a small atomic weight and has a large ionization energy.
It is considered that when a compound containing a transition metal is used as an active material for negative electrode of lithium batteries, the compound is reduced to a metal, resulting in deterioration of reversibility.
This will be explained taking iron as a transition metal and iron disulfide as a compound.
The electrode reaction of iron disulfide has been studied in thermal batteries, and it is reported that the reaction is represented by the following formula 1 (T. Tomczuk, B. Tani, N. C. Otto, M. F. Roche, and D. R. Vissers, J. Electrochem., vol. 129, p. 925 (1982)). ##STR1##
That is, the iron sulfide is reduced finally to metallic iron by the reduction reaction in the lithium batteries.
It is reported as to the electrode reaction of iron sulfide that a reaction reversible to some extent takes place at about 2V (R. Brec, A. Dugast, and A. le Mehaute, Mater. Res. Bull, vol. 15, p. 619 (1980) hereinafter referred to as "reference 1").
On the other hand, the potentials at which the reactions in the formula 1 take place are about 2.3 V for the reaction (1), about 2.0-1.5 V for the reaction (1'), and about 1.5 V for the reaction (2) as shown in FIG. 1 of reference 1.
In order to use iron sulfide as an active material for negative electrode of lithium batteries, it is necessary to use a reaction in the area showing a lower potential in the formula 1, the reaction (2). However, the metallic iron produced by reduction hardly reversibly returns to iron sulfide by subjecting it to oxidation, and the reversibility in the area of (2) is especially low. This decrease of reversibility is especially a serious problem when iron sulfide is used as an active material for negative electrode.
Production of metallic iron by the reduction reaction in case of using common liquid electrolytes or molten salt electrolytes is caused by a competitive reaction when a large current is passed, even if degree of the reduction is low. The production of metallic iron conspicuously occurs especially when degree of the reduction of iron sulfide is high, for example, when iron sulfide is further reduced from Li.sub.2 FeS.sub.2 in the area of reaction (2) in the formula 1.
The above explanation is made of using iron as a transition metal element, but a reaction of the following formula 2 also takes place using cobalt or nickel as a transition metal element (S. K. Preto, Z. Tomczuk, S. von. Winbush, and M. F. Roche, J. Electrochem. Soc., vol. 130, p. 264 (1983)). In both the cases, the compounds are reduced to metal, and hence do not show excellent performance as an active material for negative electrode of secondary batteries. EQU CoS.sub.2.rarw..fwdarw.Co.sub.3 S.sub.4.rarw..fwdarw.Co.sub.x S.sub.9.rarw..fwdarw.Co EQU NiS.sub.2.rarw..fwdarw.NiS.rarw..fwdarw.Ni.sub.7 S.sub.6.rarw..fwdarw.Ni.sub.3 S.sub.2.rarw..fwdarw.Ni 2
When the transition metal element is titanium, a reaction of the following formula 3 takes place (D. W. Murphy and J. N. Carodes, J. Electrochem. Soc., vol. 126, 349 (1979) hereinafter referred to as "reference 2"). It is reported that the reversibility of this reaction is improved by carrying out the reaction only in the area of lower potential. However, as can be seen from FIG. 4 of the reference, the reversibility is insufficient for practical secondary batteries. EQU 2Li+TiS.sub.2.rarw..fwdarw.TiS+Li.sub.2 S 3
Furthermore, reference 2 discloses that a battery of Li.sub.2 TiS.sub.2 /Li.sup.+ /TiS.sub.2 type which uses a lithium-containing transition metal sulfide as an active material for negative electrode can be a battery having an operating voltage of 1.5-1.9 V. However, as shown in FIG. 4, since the reversibility of Li.sub.2 TiS.sub.2 as an active material for negative electrode is poor, the reversibility must be further improved for obtaining practical batteries.
Reference 2 also makes mention of the reversibility of LiCrS.sub.2. However, it is disclosed that this electrode can reversibly act in the area of the potential being about 2 V and Li/Cr being 0.3, and the reaction in the lower potential area (0.7 V) which is preferred as an active material for negative electrode is irreversible. As for VS.sub.2, it is concluded there that reversibility of the electrode reaction is also poor as shown in FIG. 2, and reversibility of VSe.sub.2 is excellent, but the reaction area is of less than 1.3 atom for vanadium.
In the above, the problems to be solved by the present invention have been explained using sulfides as chalcogenides, but selenides also suffer from the same problems, and the reaction in the lower potential area (0.25 V) as seen for NbSe.sub.2 is also irreversible as shown in reference 2.